The cochlea of the human ear contains hair cells that are essential to the perception of sound. These hair cells are found along substantially the full length of the spiral path followed by the cochlea. Sound vibrations distort certain structures of the cochlea which in turn distort the hair cells. It is believed that such distortion initiates electrical impulses in the hair cells. These impulses are conveyed to the fibers of the auditory nerve and ultimately to the brain.
Some instances of human hearing loss are attributed to extensive destruction of the hair cells. When this occurs, though the structures of the cochlea may otherwise be substantially intact, and the auditory nerve may be partially or completely intact, the auditory response is significantly impaired or non-existent.
To solve this problem, implants have been developed that can directly stimulate the auditory nerve in an individual with such hearing damage. These implanted hearing devices are designed to produce useful hearing sensations to persons with severe to profound nerve deafness by electrically stimulating nerves inside the inner ear. In the past 20 years cochlear implants have helped thousands of people with hearing impairments. In fact, recent research at the University of Michigan estimates that approximately 100,000 people worldwide have received cochlear implants.
In a traditional cochlear implant system, a microphone acquires sound from the environment. The sound is then selectively filtered by a speech processor, using various filter bank strategies such as Fast Fourier Transforms, to divide the signal into different frequency bands. Once processed, the signal is then sent to a transmitter, a coil held in position by a magnet placed behind the external ear. This transmitter sends the processed signal to the internal device by electromagnetic induction. Embedded in the skull, behind the ear is a receiver which converts the signal into electric impulses and sends them through an internal cable to electrodes. Conventional cochlear implants are made of multiple platinum electrodes or similar conductive material, connected to platinum wire and embedded in a silicone body. These electrodes then act to stimulate the auditory nerve fibers by generating an electric field when the electrical current is routed to them.
There are several shortcomings to the conventional implant. First, its operation is very different from the natural hearing mechanism of the ear. For example, the conventional electrode assembly cannot stimulate auditory nerve fibers throughout the full length of the basilar membrane. In effect, because a finite number of electrodes are involved, stimulation is limited to a certain number of points. In conventional cochlear implants, the function of 1,000 inner hair cells and 30,000 auditory neurons is instead generated with only 12 to 22 channels. Thus, the full frequency spectrum of human perceptible audio is coarsely reconstructed using 12-22 frequency bands. This accounts for reported limitations regarding sound frequencies that a user of the implant can perceive.
Another significant disadvantage of the conventional cochlear implant is that the installation of the cochlear implant can damage cochlear structures. During initial insertion, for example, the basilar membrane may be injured. Furthermore, additional damage may result inside the cochlea when the endolymph is perturbed. The resulting damage to the cochlear structure, including the basilar membrane, may make replacement of the implant, or substitution of an improved implant that may be developed in the future, difficult, if not impossible, and may cause permanent loss of residual hearing.
Short (10 mm) hybrid cochlear implants may be placed in the high frequency domain of the cochlea in patients with residual hearing in the low frequency range. Obviously, this strategy cannot be used in a patient with normal hearing in the high frequency range, having poor or no hearing of low frequency sounds. For these patients, a long cochlear implant that does not damage the basilar membrane is needed to completely preserve residual hearing while stimulating that part of the cochlea that encodes high frequency sounds.
Conventional techniques such as intra-operative and post-hoc imaging using low resolution computer tomography are insufficient to prevent damage to the basilar membrane during insertion.
Further, missing from the art is a cochlear implant having an array of electrodes that achieve a finer frequency resolution across a wider band. Also missing from the art is a system and method for implanting the cochlear implant without causing injury to the basilar membrane.